Ethernet is a well known network protocol. See the IEEE specification 802.3 (the subject matter of which is incorporated herein by reference) for further background information on Ethernet. Ethernet is well suited for transferring large packets of information at spaced intervals. Information may, for example, be accumulated into a large packet and then sent in a relatively large packet from one Ethernet node to another Ethernet node. Ethernet can therefore be said to be a “bursty” network protocol.
Some types of information, such as the information in a typical telephone conversation, do not lend themselves to being accumulated over time and then being transmitted as a single large packet. In a telephone conversation, speech information should be passed from speaker to listener without significant delay because the listener may use that speech information to formulate a response. Accordingly, there is not time for large packets of information to be accumulated. Frequent transmissions of small packets of information over the network is required. Ethernet is not well suited to this “nonbursty” type of information transfer.
There are, however, communication protocols (called isochronous protocols) which are suited for communication of such “nonbursty” information. Circuit switching and time division multiplexing (TDM) techniques are employed to divide a communication medium into a number of consecutive frames, each frame including a number of time slots. A first telephone conversation may, for example, be allocated a first slot of each frame whereas a second telephone conversation may be allocated a second slot of each frame. Because telephone information for each conversation is received each frame, the “nonbursty” information of the telephone conversations is communicated without significant delay.
Isochronous networks may also be made to carry “bursty” information. Telephone companies use an information framing protocol known as “HDLC” to frame information (“bursty” and/or “nonbursty”) for isochronous communication over a standard digital telephone line (an example of which is Primary Rate ISDN or “PRI”). HDLC is part of a more encompassing protocol called “X.25” See the document ISO/IEC 3309, 1991 (the subject matter of which is incorporated herein by reference) for additional information on the HDLC protocol.
FIG. 1 (Prior Art) shows an interconnection of networks. Telephone information passes to and from telephones 1 and 2 over PBX (Private Branch Exchange) lines 3 and 4, respectively, to a local PBX 5. The local PBX 5 is coupled to a central office/exchange 6 (typically operated by a telephone company) via one or more PRI lines 7. “Nonbursty” telephone conversation information passes over this structure.
“Bursty” information such as video information and large computer files, on the other hand, passes over another structure. A first Ethernet network 8 having a plurality of workstations and a file server and an Ethernet hub is coupled to a second Ethernet network 9 via two Ethernet lines 10, 11 and an Ethernet hub/router 12. The file server of a network may, for example, store video data which can be accessed and displayed by the workstations of the network. Lines 10 and 11 are logically two different Ethernet lines. Hub/router 12 is coupled to the central office/exchange 6 via an isochronous link 13 such as a PRI line. Information is passed over link 13 using the HDLC protocol. The dots on selected workstations indicate video cameras.
A video camera of a workstation in the first Ethernet network can therefore capture video information and store that information in the file server of the first Ethernet network 8. A workstation in the second Ethernet network 9 can then access that information over Ethernet lines 10 and 11 via hub/router 12 and display that information. A workstation can also receive HDLC packaged “bursty” information (such as the yellow pages in graphic form) from the central office/exchange 6 via isochronous link 13.
There exists, however, another information packaging protocol known as asynchronous transfer mode (hereinafter “ATM”). See the document “ATM User-Network Interface Specification”, Version 3.0 (the subject matter of which is incorporated herein by reference) for additional information on the ATM protocol. Although it is envisioned that ATM will eventually replace HDLC, it is likely that significant numbers of ATM and HDLC data communication services will coexist for a significant period of time. It would therefore be desirable to provide network node hardware capable of both ATM and HDLC communication. Furthermore, a user using the structure of FIG. 1 would likely have a telephone on his/her desk in addition to a workstation. Accordingly, a PBX line would extend onto the user's desk for coupling to the telephone and an Ethernet line would also extend onto the user's disk for coupling to the workstation. It would be desirable to eliminate one of these two lines so that the workstation could receive both “bursty” Ethernet information and “nonbursty” telephone information over a single line.